The present invention relates to the electrical speed control and medical diagnosic arts. It finds particular application in controlling acceleration and deceleration of a rotating anode found in x-ray tubes used in conjunction with computed tomography scanners, and will be described with reference thereto. It is to be appreciated, however, that the invention may also find application in digital x-ray scanners, conventional x-ray, other x-ray medical and non-medical devices, other motor and rotation speed control applications, and the like.
A conventional x-ray tube includes a thermionic filament cathode and a rotating anode which are encased in an evacuated envelope. A heating current, commonly on the order of 2-5 amps, is applied through the filament to create a surrounding electron cloud. A high potential, e.g. 50-150 kilovolts, is applied between the filament and the anode to accelerate the electrons from the cloud to an anode target area. This acceleration of electrons causes a tube or anode current which is commonly on the order of 5-200 milliamps. To inhibit the target area from overheating, the anode rotates at a high operating speed during x-ray generation. When no x-rays are being generated, the anode may be allowed to decelerate.
In a computed tomography scanner, an x-ray tube is disposed radially outward of an imaging region and opposite to a plurality of sensors. Commonly, a fan beam of x-rays is generated by the tube, passes through a subject, and impinges on the sensor array. As the x-ray tube is rotated about the subject, the sensors generate a series of views from which an image is reconstructed. After each series of views from which an image is reconstructed. After each series of views or scan, the x-ray tube stops generating x-rays. If another scan is to follow, the anode may be kept rotating at full speed between scans. Historically, however, after most scans, the anode was permitted to slow or stop. Thus, the x-ray tube anode goes through frequency acceleration/deceleration cycles.
The rotating anode of an x-ray tube has one or more natural resonant frequencies, i.e. angular velocities at which the anode vibrates excessively. The vibration may damage the anode, as well as shorten the bearing life. It is accordingly desirable to minimize the time spent at the resonant angular velocity during acceleration and deceleration. During deceleration, the rotating anode is braked until it is below the resonant angular velocity. Thereafter, the anode coasts to a complete stop.
Conventional high speed drive circuits for x-ray tube anode motors include large, heavy, SCR type inverters, which feed three leads of a two winding stator. Two leads, a run lead and a quadrature lead, are actively driven. The third lead, a common lead, serves as a return line to the others. The windings were traditionally placed in a fixed split-capacitor drive mode.
The quadrature (or out-of-phase winding) is operated near its resonance frequency to accomplish a current phase shift between the windings. Because a wide range of x-ray tubes with widely varying motor characteristics may be used as a load, phase shift and ampere turn matching of the winding is often far from optimum. Multiple capacitors are commonly switched in and out to improve the matching for different stators and operating frequencies. Additionally, as the current level in the windings changes, the windings change temperature. The phase and relative magnitude of the current through the windings change correspondingly. This causes excessive current to be necessary to accelerate the rotor to its operating speed in the required time and to maintain the correct operating speed.
An additional problem is encountered when driving such two-coil induction motor driven x-ray tubes with a half-bridge driver. When utilizing a half-bridge driver, a single, common lead forms an input to both coils of the stator. Since the common lead is connected to a neutral point in the driver power supply, it is not possible to drive each coil independently at the maximum DC potential available.
To alleviate the above-noted phase and matching problems, two independent inverters can be provided; one for each winding. However, such dual-inverter systems are costly and bulky. Further, the dual inverter systems cannot operate at the low frequencies which are necessary to provide optimum AC braking, unless large output transformers are provided. To avoid the large output transformer, a DC brake voltage is often coupled, via a contactor, to the windings of the tube. While this is generally effective for braking, it does not provide good control over rotor speed during braking, particularly immediately before entering the coasting mode.
The present invention contemplates a new and improved method and apparatus for controlling acceleration/deacceleration of an anode rotor in an x-ray tube, which overcomes the above-referenced problems and others.